In applicant's prior U.S. Pat. No. 5,159,993 dated Nov. 3, 1992 and entitled SELF-RAISING WORK PLATFORM ASSEMBLY, there is described a pair of towers adapted to be erected alongside a building structure and anchored thereto, elongated sleeves surrounding said towers and guided for up and down movement therealong and work platforms extending between the towers and supported by the sleeve members. Power operated chain blocks are used for raising and lowering the sleeve members and platforms to the required working level. Such chain blocks must be attached to the tower at successively higher levels each time it is desired to raise the work platforms and the workmen must climb the tower to attach the chain blocks thereto. This procedure is sometimes considered dangerous for the workmen. The same chain block system is used in applicant's U.S. patent application No. 07/775,909 filed on Oct. 15, 1991, allowed on Dec. 9, 1992 and entitled SELF-RAISING CANTILEVER TYPE WORK PLATFORM ASSEMBLIES.
Such a disadvantage is overcome by the raising system described in U.S. Pat. No. 4,809,814 dated Mar. 7, 1989, inventor: Jean St-Germain, and entitled SCAFFOLDING. Referring to FIGS. 8 and 9 of this patent, it is seen that each tower is formed with a plurality of vertically equally spaced rungs 38 successively engaged by a hook 116 which is attached at the top end of a piston rod 112 of a hydraulic ram 106 pivoted at 110 to the platform 42 to be raised along the tower. Upon the return contraction stroke of the ram 106, the platform is held by the latching levers 84 engaging a tower rung. This system works well but it sometimes happens that the hook 116 fails to properly engage a tower rung. This was found to be produced by the fact that the hook 116 is not properly guided since only the lower end of ram 106 is attached and since the ram must have a certain length to permit sufficient vertical stroke of the hook 116 to move from one rung to the next one.
This malfunction may cause serious injuries to the workers which are high up above ground. Some self-raising work platform assemblies use a rack and pinion driving system to raise and lower the platform along the towers. Such a rack and pinion system requires a relatively high precision in the assembly of the parts and is quite expensive to build and operate.